In a flat-panel display device, e.g., a liquid crystal display (LCD), an organic light-emitting diode (OLED) display or an inorganic electroluminescent (EL) display, TFT is generally used as a switching element to control pixels or used as a drive element to drive the pixels. The TFTs can be divided into amorphous silicon (a-Si) TFTs and polysilicon (Poly-Si) TFTs according to the properties of a silicon membrane used for an active layer. Compared with the a-Si TFT, a Poly-Si TFT has the characteristics of higher electron mobility, lower drain current and the like. Thus, the display manufactured via the Poly-Si TFT could have higher resolution and faster response speed.
The process for preparing the Poly-Si membrane can be divided into two categories. One involves a high-temperature process. In the preparation process, the temperature is higher than 600 centigrade, and a substrate is made from expensive quartz. The other involves a low-temperature process. In the whole processing technique, the temperature is lower than 600 centigrade, and the substrate may be made from cheap glass. Thus, the LTPS technology has gradually become the mainstream technology in the research and development of TFTs and in place of the a-Si technology. In the preparation of LTPS, the problem of Poly-Si crystallization has always been the focus of research in the field of LTPS.
The mature LTPS preparation process in the current industry mainly comprises for example solid phase crystallization (SPC), metal-induced lateral crystallization (MILC) and excimer laser crystallization (ELC). The ELC technology is commonly used in the crystallization of a-Si in the industry due to high electron mobility and productivity of products thereof. ELC involves applying high-power laser beams to act on the surface of an a-Si membrane to be crystallized. Because silicon materials have strong ultraviolet light absorption capability, the laser beams can drive the surface of the a-Si membrane to reach the high temperature of more than 1,000 centigrade and be in a molten state within a very short time period (about 50 ns to 150 ns). After the laser pulse is stopped, the a-Si in the molten state is cooled and crystallized into Poly-Si. The Poly-Si membrane prepared by an ELC process has the advantages of large crystal grains, good spatial selectivity, high doping efficiency, low intracrystalline imperfection, good electrical properties and high mobility, and is an LTPS membrane with an optimal overall performance currently. However, the Poly-Si membrane prepared by an ELC process also has defects. That is to say, the size of the crystal grains is sensitive to the laser power, and the prepared Poly-Si membrane has poor uniformity. Thus, products (e.g., TFTs) prepared by the Poly-Si membrane have large performance difference.
In summary, the Poly-Si membrane prepared by the ELC technology currently has poor uniformity, and hence the performances of the products prepared by the Poly-Si membrane can be disadvantageously affected.